1. Field of the Invention
The present invention relates to variable resistance devices and more particularly to an improved switch of the type used for keyboard switching arrays.
2. Description of the Prior Art
Carbon pile resistors which comprise a plurality of rigid carbon buttons or disks have been employed as variable resistance elements. Such disks have been arranged in a stack between terminals in a series circuit. These variable resistance elements employ the application of a relatively large pressure to the stack of carbon disks to vary the surface resistance between the disk and thus between the terminals. The range of resistance of carbon pile resistors is limited and the carbon disks require substantial pressures to obtain desired resistance variations. The required pressure is supplied through devices such as screws or other force enhancing mechanisms. Carbon pile resistance elements are not practical for use where a light finger pressure is the actuating force, thus, have not been known to be employed in switches.
Miniature touch control switches and touch control panels are known. Such prior art touch control devices sense the closing of a circuit or the change of capacitance resulting from the touching of a switch element. The presence or absence of the finger may be sensed through a scanning device or may be sensed as a change in capacitance of an associated circuit when touched by a finger. Scanning devices usually employ a light intercept principal which requires complex sensing and logic circuitry. Capacitative sensing devices also employ relatively complex circuitry as well as requirement for continuous electrical current.
Membrane switches differ from the present invention. In one form of membrane switch two electrically conductive elements are separated by a very thin dielectric insulating separator having an aperture therein. The two electrically conductive elements, usually in flexible sheet form, are physically connected through the aperture by the application of pressure to one of the sheets. The problem with such switches is that surface resistance of the sheets and/or distortion of the sheets changes the resistance appreciably.
Capacitive touch switches employ a more complex structure. Two or more capacitors are arranged in a series circuit. Each capacitor has its plates separated from each other by a dielectric insulator. The capacitors are arranged in a logic circuit so that a current from a clock signal is applied as an input to one side of the capacitor and sensed at the other side as an output level. When the mutual or common plate of the capacitors is touched, the output signal therethrough is effectively grounded, thus, no signal is raised on the output line. The lack of a signal is employed in the logic circuits to detect a touch of the switch. The problem with such switches is that grounds and other malfunctions are detected as a touch signal. Such circuits always draw current even though not in use.
It would be desirable to provide a switch for use in electronic circuits which has no contacts to wear or change resistance through oxidation. Further, it would be desirable to provide a switch which operates similar to a touch switch and/or can be made to respond to a predetermined force or a predetermined displacement.